Well, I'll show you an example program. So gets() reads a string from the input. So you'll want to provide some sort of prompt with printf. So,
printf("Enter a number: ");
Then you'll want to retrieve a number with gets...
char number[5];
gets( &number );
Then you'll want to change that to an integer, which should look something like...
int numberInteger = atoi(number)
You'll still have some stuff to fill in, but after it's a number you'll be able to use it in calculations. Make sense?